Phenolphthalein was commonly used as a laxative for most of the 20th century. The use of phenolphthalein in laxatives has decreased since 1997 when the United States Food and Drug Administration (FDA) proposed to withdraw its classification as an over-the-counter drug (21 CFR, Part 310). Phenolphthalein has been previously evaluated in 2-year carcinogenicity studies by the National Toxicology Program (1996). The major route of human exposure to phenolphthalein is via ingestion, dermal contact, and inhalation of contaminated air originating from process units manufacturing the compound. In this study, the carcinogenic effects of phenolphthalein were studied in the haploinsufficient p16Ink4a/p19Arf mouse model as an ongoing goal of the NTP is to seek model systems for toxicology and carcinogenesis studies, especially those that can provide mechanistic information relative to understanding an agent's mode of action. Male and female haploinsufficient p16Ink4a/p19Arf mice were exposed to phenolphthalein (greater than 97% pure) in feed for 27 weeks. Genetic toxicology studies were conducted in mouse peripheral blood erythrocytes.
27-Week Study in Mice
Groups of 15 male and 15 female mice were exposed to 0, 200, 375, 750, 3,000, or 12,000 ppm phenolphthalein (equivalent to average daily doses of approximately 35, 65, 135, 540, and 2,170 mg phenolphthalein/kg body weight to males and 50, 90, 170, 680, 2,770 mg/kg to females) in feed for 27 weeks. Survival of all exposed groups of male and female mice was similar to that of the control groups. Mean body weights of males in the 12,000 ppm group were less than those of the control group after week 11. No differences in feed consumption were noted between exposed and control groups.
Atypical hyperplasia of the thymus, a premalignant change of chemically induced thymic lymphoma, occurred in exposed males and females, and the incidence was significantly increased in 12,000 ppm females. Atrophy of the seminiferous tubules in the testis, hyperplasia of the testicular interstitial (Leydig) cells, and epididymal hypospermia occurred in most 3,000 and 12,000 ppm males. Additionally, the left and right testis weights, the left epididymis weights, sperm motility, the numbers of spermatid heads per testis, and sperm heads per cauda and per gram cauda epididymis were generally significantly less in 3,000 and 12,000 ppm males than in the control group. The incidences of nephropathy were significantly increased in 3,000 and 12,000 ppm males; incidences of hypertrophy of renal tubules were significantly increased in males receiving 750 ppm or greater. Hematopoietic cell proliferation of the spleen occurred in all 12,000 ppm males, and the incidences of this lesion were significantly increased in 375, 750, and 12,000 ppm females.
Genetic Toxicology
The frequency of micronucleated erythrocytes was assessed at four time points during the 27-week study in male and female haploinsufficient p16Ink4a/p19Arf mice. Significant concentration-related increases in micronucleated cells were observed at all time points in male and female mice.
Conclusions
Under the conditions of this 27-week feed study, there was no evidence of carcinogenic activity of phenolphthalein in male or female haploinsufficient p16Ink4a/p19Arf mice exposed to 200, 375, 750, 3,000, or 12,000 ppm. Because this is a new model, there is uncertainty whether the study possessed sufficient sensitivity to detect a carcinogenic effect.
Phenolphthalein induced atypical hyperplasia, a preneoplastic lesion of the thymus, in male and female mice, hematopoietic cell proliferation of the spleen in male and female mice, and toxicity to the kidney and reproductive system in male mice.